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- `ExpressionNice`: a subclass of `sage.symbolic.expression.Expression` with enhanced display of callable symbolic expressions

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Internally, `ScalarField`'s are described by their coordinate representations in various charts, which are implemented as a dictionary of `CoordFunction`'s, with the charts as keys.

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At the moment, there is only one concrete class for coordinate functions: `CoordFunctionSymb` (functions described by symbolic expressions of the coordinates), but in the future there should be numerical coordinate functions (hence the abstract base class `CoordFunction`).